Flow rate measurement method and apparatus

ABSTRACT

Method and apparatus capable of accurately measuring a flow rate using Gaussian quadrature even under a small number of measurement points are provided. An average for the y-coordinate of values of normal-directional component with respect to the cross-section represented by v z0 (x, y) among an estimated flow velocities in a flow passage cross-section is used as V z0 (x), and a weighting function of Gaussian quadrature is set to V z0 (x)L(x), where L(x)=y max (x)−y min (x). Further, an average for y of values of normal-directional component with respect to the cross-section of an actual flow velocity represented by v z (x, y) is used as V z (x), and an integrand is set to V z (x)/V z0 (x). These values are used to determine an approximate value of a flow rate according to Gaussian quadrature. In addition, a method of virtually offsetting zero-point of the flow velocity distribution to apparently avoid a problem such as reverse flow is provided.

TECHNICAL FIELD

The subject technology relates to a method and apparatus for measuring a flow rate with a high degree of accuracy in a flow passage having difficulty in sufficiently achieving flow-straightening, typically in a liquid rocket engine or other ground testing facilities, and the like.

BACKGROUND

Generally, a volume flow rate can be defined as a volume of a fluid flowing through a flow passage cross-section within a time unit. For example, a flow rate Q of a fluid flowing through a flow passage pipe arrangement 1 illustrated in FIG. 1 is represented as an integrated value of normal-directional components with respect to a flow passage cross-section of flow velocities at respective points on the flow passage cross-section, as follows:

$\begin{matrix} {{\begin{matrix} {Q = {\int_{S}\ {{s}\; v_{z}}}} \\ {= {\int_{x_{\min}}^{x_{\max}}\ {{x}{\int_{y_{\min}{(x)}}^{y_{\max}{(x)}}\ {{y}\; {v_{z}\left( {x,y} \right)}}}}}} \\ {= {\int_{x_{\min}}^{x_{\max}}\ {{x}\; {V_{z}(x)}{L(x)}}}} \end{matrix}{{where},}}} & (1) \\ {{V_{z\;}(x)} = \frac{\int_{y_{\min}{(x)}}^{y_{\max}{(x)}}\ {{y}\; {v_{z\;}\left( {x,y} \right)}}}{{y_{\max}(x)} - {y_{\min}(x)}}} & (2) \\ {{L(x)} = {{y_{\max}(x)} - {{y_{\min}(x)}.}}} & (3) \end{matrix}$

In the Formulas (1) to (3), S represents a flow passage cross-section, wherein a position in the flow passage cross-section S is represented by (x, y). v_(z) represents a normal-directional (z-directional) component of a flow velocity of the fluid with respect to the flow passage cross-section S, and v_(z)(x, y) represents a normal-directional component at a position (x, y) of a flow velocity of the fluid at the position (x, y) on the flow passage cross-section S. y_(min)(x) and y_(max)(x) represent, respectively, a minimum value and a maximum value of the y-coordinate when the x-coordinate is fixed on the flow passage cross-section S, and x_(min) and x_(max) represent, respectively, a minimum value and a maximum value of the x-coordinate on the flow passage cross-section S. Additionally, in FIG. 1, the flow passage cross-section S₁ is a plane orthogonal to an axis of the flow passage pipe arrangement 1, but even if the flow passage cross-section is inclined with respect to the axis, or is a curved surface, the flow rate can be represented by the above Formulas (1) to (3).

As is clear from the above Formula (2), when x_(i) as the x-coordinate is set to a certain point, V_(z)(x_(i)) represents an average of values of v_(z)(x, y) for respective points (x, y), on a region in the flow passage cross-section S in which the x-coordinate is equal to x_(i), namely on a line segment AB (FIG. 2), and the V_(z)(x_(i)) can be measured by, for example, an ultrasonic wave method.

In one example, as illustrated in FIG. 3, assume that two points each shifted from a respective one of points A, B in a z-direction by a given distance are represented, respectively, as A* and B*. In this case, by utilizing a phenomenon that an apparent difference in sound velocity corresponding to flow velocity occurs between when an ultrasonic wave is propagated in a direction from the point A* to the point B* and when the ultrasonic wave is propagated in a direction from the point B* to the point A*, an average of values of v_(z)(x, y) between A* and B* can be derived from respective propagation times in the two directions. Generally, the average value can be deemed as an average flow velocity V_(z)(x_(i)) on the line segment AB.

More specifically, as illustrated in FIG. 4, an ultrasonic element 2 a configured to transmit an ultrasonic wave from the point A* and an ultrasonic element 2 b configured to transmit an ultrasonic wave from the point B* are disposed on the flow passage pipe arrangement 1, to measure a propagation time of the ultrasonic wave transmitted from the ultrasonic element 2 a until it is received by the ultrasonic element 2 b, and a propagation time of the ultrasonic wave transmitted from the ultrasonic element 2 b until it is received by the ultrasonic element 2 a. In a simple example, as described in the Patent Document 1, on an assumption that a propagation time of the ultrasonic wave transmitted from the ultrasonic element 2 a to the ultrasonic element 2 b until it reaches the ultrasonic element 2 b via a path having a length L, and a propagation time of the ultrasonic wave transmitted from the ultrasonic element 2 b to the ultrasonic element 2 a until it reaches the ultrasonic element 2 a via the path are represented, respectively, as t₁ and t₂, the average of the values of v_(z)(x, y) can be determined by the following Formula (4) (where an angle between the ultrasonic wave transmission and receipt path and the z-direction is represented as θ (theta)).

$\begin{matrix} {\overset{\_}{v_{z}\left( {x,y} \right)} = {\frac{L}{2\mspace{14mu} \cos \mspace{14mu} \theta}\left( {\frac{1}{t_{2}} - \frac{1}{t_{1}}} \right)}} & (4) \end{matrix}$

Generally, this average value can be used as a value of V_(z)(x_(i)).

In practice, V_(z)(x_(i)) can be measured at discrete positions. Thus, in many cases, it is necessary to calculate an approximate value of Q using numerical integration. Further, in the case of performing measurement using the pair of ultrasonic elements as illustrated in FIG. 4, it is conceivable to arrange the pairs of ultrasonic elements on the flow passage pipe arrangement in a number corresponding to the number of x_(i) points to be measured. In this case, however, there is a limit on the number of measurement points which can be arranged, and thus it is necessary to use a calculation technique capable of maintaining good accuracy even under a small number of measurement points.

A method for deriving an approximate value of the flow rate Q represented by the Formula (1), from the discretely measured flow velocities, by Gaussian quadrature (Gauss-Legendre type, Chebyshev type, and Lobatto type) is disclosed in the Patent Document 2. The method disclosed in the Patent Document 2 is configured to determine an approximate value Q_(N) of the flow rate Q by the following Formula, using Gaussian quadrature under a condition that an integrand is set to V_(z)(x)L(x) and a weighting function is set to 1 (constant):

$\begin{matrix} {Q_{N} = {\sum\limits_{i = 1}^{N}\; {w_{i}{V_{z}\left( x_{i} \right)}{{L\left( x_{i} \right)}.}}}} & (5) \end{matrix}$

In this Formula, x_(i) and w_(i) represent, respectively, a node and a coefficient of the Gaussian quadrature. The Formula (5) is calculated using discrete flow velocity values of V_(z)(x_(i)) measured at respective nodes of x=x_(i).

The Patent Documents 3 and 4 disclose a method for deriving an approximate value of flow rate of a fluid flowing through a flow passage of a circular pipe having a radius R, from a discretely measured flow velocities. In the flow passage of the circular pipe having a radius R, when the x-coordinate is defined on a flow passage cross-section, L(x) in the Formula (3) is expressed as follows:

$\begin{matrix} {{L(x)} = {2R{\sqrt{1 - \left( \frac{x}{R} \right)^{2}}.}}} & (6) \end{matrix}$

Utilizing this, in the methods disclosed in the Patent Documents 3 and 4, on an assumption that the weighting function is set as follows:

$\begin{matrix} {\sqrt{1 - \left( \frac{x}{R} \right)^{2}},} & (7) \end{matrix}$

and on an assumption that the integrand is set to 2RV_(z)(x), the nodes x_(i) and the coefficients w_(i) are calculated in accordance with Gaussian quadrature (Gauss-Chebyshev type). The approximate value Q_(N) of the flow rate Q is determined by the following approximation formula, using discrete values of the flow velocity V_(z)(x_(i)) measured at respective nodes of x=x_(i);

$\begin{matrix} {Q_{N} = {\sum\limits_{i = 1}^{N}{w_{i}2{{{RV}_{z}\left( x_{i} \right)}.}}}} & (8) \end{matrix}$

In the above conventional techniques, the flow velocity distribution itself is included in the integrand. Thus, there is a problem that, for example, in a situation where the flow velocity distribution is largely distorted, a higher-order quadrature (namely, a larger number of measurement points) is required to obtain an accurate approximate value.

LIST OF PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP 2001-208584A

Patent Document 2: U.S. Pat. No. 3,564,912A

Patent Document 3: U.S. Pat. No. 3,940,985A

Patent Document 4: JP 551-129261A

SUMMARY Examples of Technical Problem

In view of the above circumstances, one or more implementations of the subject technology provide a method and apparatus for accurately measuring a flow rate even under a small number of measurement points.

Examples of Solution to the Technical Problem

One or more implementations of the subject technology provide a method for measuring a flow rate Q of a fluid flowing through a flow passage, the flow rate Q being represented by the following Formula:

$\begin{matrix} \begin{matrix} {Q = {\int_{S}{{sv}_{z}}}} \\ {= {\int_{x_{m\; i\; n}}^{x_{m\; {ax}}}{{x}{\int_{y_{m\; i\; n}{(x)}}^{y_{{ma}\; x}{(x)}}{{{yv}_{z}\left( {x,y} \right)}}}}}} \\ {= {\int_{x_{m\; i\; n}}^{x_{m\; i\; n}}{{{{xV}_{z}(x)}}{L(x)}}}} \end{matrix} & (9) \\ {{where},} & \; \\ {{V_{z}(x)} = \frac{\int_{y_{m\; i\; n}{(x)}}^{y_{{ma}\; x}{(x)}}{{{yv}_{z}\left( {x,y} \right)}}}{{y_{{ma}\; x}(x)} - {y_{m\; i\; n}(x)}}} & (10) \\ {{L(x)} = {{y_{{ma}\; x}(x)} - {y_{m\; i\; n}(x)}}} & (11) \end{matrix}$

(where: S represents a flow passage cross-section, wherein a position within the flow passage cross-section S is represented as (x, y); v_(z) represents a normal-directional (z-directional) component with respect to the flow passage cross-section S of a flow velocity of the fluid, wherein v_(z)(x, y) represents a normal-directional component at a position (x, y) of a flow velocity of the fluid at the position (x, y) on the flow passage cross-section S; and y_(min)(x) and y_(max)(x) represent, respectively, a minimum value and a maximum value of the y-coordinate when the x-coordinate is fixed on the flow passage cross-section S, wherein x_(min) and x_(max) represent, respectively, a minimum value and a maximum value of the x-coordinate on the flow passage cross-section S, wherein these apply to formulas described below),

the method comprising the steps of:

determining a node x_(i) and a coefficient w_(i) for Gaussian quadrature (where i represents a natural number of 1 to N, where N represents an arbitrary natural number equal to or greater than 1), using a weighting function V_(z0)(x) L(x) represented by,

V_(z0)(x)L(x)   (12),

where

$\begin{matrix} {{{V_{z\; 0}(x)} = \frac{\int_{y_{m\; i\; n}{(x)}}^{y_{{ma}\; x}{(x)}}{{{yv}_{z\; 0}\left( {x,y} \right)}}}{{y_{{ma}\; x}(x)} - {y_{m\; i\; n}(x)}}},} & (13) \end{matrix}$

where v_(z0)(x, y) represents an estimate value of a normal-directional component of a flow velocity at each position (x, y) within the flow passage cross-section S;

determining V_(z)(x_(i)) and V_(z0)(x_(i)) for respective nodes x_(i); and

determining an approximate value Q_(N) of the flow rate Q, the approximate value Q_(N) being represented by using the following Formula:

$\begin{matrix} {Q_{N} = {\sum\limits_{i = 1}^{N}{w_{i}{\frac{V_{z}\left( x_{i} \right)}{V_{z\; 0}\left( x_{i} \right)}.}}}} & (14) \end{matrix}$

As described above, the method of the subject technology is characterized by formulating a weighting function V_(z0)(x)L(x) using an estimated flow velocity distribution on the flow passage cross-section S (which may be preliminarily derived by a theoretic calculation, a simulation, a preliminary experiment, or the like), and determining the approximate value of the flow rate by Gaussian quadrature. In this case, the integrand is expressed as:

$\begin{matrix} {\frac{V_{z}(x)}{V_{z\; 0}(x)}.} & (15) \end{matrix}$

This represents a ratio of an average of values of actual flow velocity distribution to an average of values of the estimated flow velocity distribution for a certain x-coordinate. When the estimated flow velocity distribution is more probable, the accuracy of approximation is increased because the integrand becomes closer to a constant function.

This method is also practicable using V′_(z0)(x) obtained by modifying the V_(z0)(x) with an appropriate offset function V_(z1)(x).

That is, the subject technology provides a method for measuring a flow rate Q of a fluid flowing through a flow passage, the flow rate Q being represented by the Formulas (9) to (11), the method comprising the steps of:

determining a node x_(i) and a coefficient w_(i) for Gaussian quadrature (where i represents a natural number of 1 to N, where N represents an arbitrary natural number equal to or greater than 1), using a weighting function V′_(z0)(x) L(x) represented by,

$\begin{matrix} {{{V_{z\; 0}^{\prime}(x)}{L(x)}},{where}} & (18) \\ {{{V_{z\; 0}^{\prime}(x)} = {{V_{z\; 0}(x)} + {V_{z\; 1}(x)}}},{{where}\mspace{14mu} {V_{z\; 1}(x)}\mspace{14mu} {is}\mspace{14mu} {an}\mspace{14mu} {offset}\mspace{14mu} {function}},{where}} & (16) \\ {{{V_{z\; 0}(x)} = \frac{\int_{y_{m\; i\; n}{(x)}}^{y_{m\; a\; x}{(x)}}{{{yv}_{z\; 0}\left( {x,y} \right)}}}{{y_{{ma}\; x}(x)} - {y_{m\; i\; n}(x)}}},} & (17) \end{matrix}$

where v_(z0)(x, y) represents an estimate value of a normal-directional component of a flow velocity at each position (x, y) within the flow passage cross-section S;

for respective nodes x_(i), determining V_(z)(x_(i)), and determining V′_(z0)(x_(i)), V_(z1)(x_(i)), and Q_(z0) and Q_(z1) each represented by a respective one of the following Formulas:

Q _(z0)=∫_(x) _(min) ^(x) ^(max) dxV _(z0)(x)L(x)   (19)

Q _(z1)=∫_(x) _(min) ^(x) ^(max) dxV _(z1)(x)L(x)   (20), and

determining an approximate value Q_(N), the approximate value Q_(N) being represented by the following Formula:

$\begin{matrix} {Q_{N} = {\frac{\sum\limits_{i = 1}^{N}{w_{i}\frac{V_{z}\left( x_{i} \right)}{V_{z\; 0}^{\prime}\left( x_{i} \right)}}}{1 + \frac{Q_{z\; 1}}{Q_{z\; 0}} - {\frac{1}{Q_{z\; 0}}{\sum\limits_{i = 1}^{N}{w_{i}\frac{V_{z\; 1}\left( x_{i} \right)}{V_{z\; 0}^{\prime}\left( x_{i} \right)}}}}}.}} & (21) \end{matrix}$

This method is based on an idea of using the offset function V_(z1)(x) to offset respective zero-points of V_(z0)(x) and V_(z)(x) to thereby introduce V′_(z0)(x) and V′_(z)(x), and then using them to apply Gaussian quadrature to a flow rate Q′ obtained based on offset zero-points, as described below. As described below, it is possible to derive the Formula (21) by a process of: introducing a flow rate Q′ obtained based on zero-points offset using V′_(z)(x) instead of V_(z)(x) in the Formula (9); applying Gaussian quadrature assigned with V′_(z0)(x)L(x) as a weighting function to the flow rate Q′ to thereby represent an approximate value Q^(′)N in the form of polynomial approximation; and representing the Q_(N) in the form of polynomial approximation using a relational formula between Q′_(N) and Q_(N).

In the above method that employs V_(z1)(x), the V_(z1)(x) is preferably selected to allow V′_(z0)(x) to be always zero or more on the flow passage cross-section S, or to be always zero or less on the flow passage cross-section S.

In the case where the sign of the weighting function is not constant due to, for example, a so-called reverse flow, it may not be necessarily possible to have a node x_(i) of Gaussian quadrature in the integral interval (a range taken by the x-coordinate on the flow passage cross-section S), so that it is possible that the measurement point x_(i) of V_(z)(x_(i)) or the calculation point x_(i) of V₀(x_(i)) cannot be implemented. Even in this case, by selecting the V_(z1)(x) in such a manner as to allow the sign of the weighting function V′_(z0)(x)L(x) to be constant in the integral interval (for example, setting an absolute value of the minimum value of V_(z0)(x) to V_(z1)(x), or by setting an absolute value of the maximum value of V_(z0)(x)x(−1) to V_(z1)(x) allows the sign of the weighting function V′_(z0)(x)L(x) to be constant), the node x_(i) can be implemented in the integral interval as a corollary of the principle of Gaussian quadrature. Thus, it becomes possible to apparently avoid the reverse flow and accurately determine the flow rate.

However, as described below, it may also be possible to have the node x_(i) in the integral interval even if the sign of the weighting function is changed in the integral interval. Therefore, it is not essential for the subject technology to select V_(z1)(x) in such a manner as to allow the sign of the weighting function to be constant as described above.

The subject technology further provides a flow rate measurement method, comprising the steps of: determining the approximate value Q_(N) according to the method provided by the subject technology as described above; and estimating an error range of the approximate value Q_(N) based on the following Formula using a second approximate value Q_(SUB) that is less reliable than the approximate value Q_(N):

$\begin{matrix} {{{{{if}\mspace{14mu} Q_{SUB}} < Q_{N}},{\frac{Q_{SUB} - Q_{N}}{2} < {Q - Q_{N}}}}{{{{if}\mspace{14mu} Q_{SUB}} > Q_{N}},{\frac{Q_{SUB} - Q_{N}}{2} > {Q - {Q_{N}.}}}}} & (22) \end{matrix}$

As described below, this method makes it possible to estimate an error of the approximate value Q_(N). As used herein, the term second approximate value Q_(SUB) which is “less reliable than the approximate value Q_(N)” means an approximate value which is assumed to have an error greater than that of the approximate value Q_(N) and which is determined by, for example, decreasing the number of measurement points N when using the method of the subject technology, or, as described below, by uniformly replacing values of the integrand of the Gaussian quadrature with a maximum or minimum value thereof, instead of individually calculating the integrand of the Gaussian quadrature for respective nodes x_(i).

The subject technology further provides an apparatus for measuring a flow rate Q of a fluid flowing through a flow passage, the flow rate Q being represented by the Formulas (9) to (11), the apparatus comprising: a Gaussian quadrature operation section for determining a node x_(i) and a coefficient w_(i) for Gaussian quadrature (where i represents a natural number of 1 to N, where N represents an arbitrary natural number equal to or greater than 1), using a weighting function V_(z0)(x) L(x) represented by the Formulas (12) and (13), where v_(z0)(x, y) represents an estimate value of a normal-directional component of a flow velocity at each position (x, y) within the flow passage cross-section S; a V_(z)(x_(i)) determining section for determining V_(z)(x_(i)) for each node x_(i) determined by the Gaussian quadrature operation section; a V_(z0)(x_(i)) determining section for determining V_(z0)(x_(i)) for each node x_(i) determined by the Gaussian quadrature operation section; and a Q_(N) determining section for determining an approximate value Q_(N) of the flow rate Q, the approximate value Q_(N) being represented by using the Formula (14).

The subject technology further provides an apparatus for measuring a flow rate Q of a fluid flowing through a flow passage, the flow rate Q being represented by the Formulas (9) to (11), the apparatus comprising: a Gaussian quadrature operation section for determining a node x_(i) and a coefficient w_(i) for Gaussian quadrature (where i represents a natural number of 1 to N, where N represents an arbitrary natural number equal to or greater than 1), using a weighting function V′_(z0)(x)L(x) represented by the Formula (18), using V′_(z0)(x) represented by the Formulas (16), (17) using an offset function V_(z1)(x), where v_(z0)(x, y) represents an estimate value of a normal-directional component of a flow velocity at each position (x, y) on the flow passage cross-section S; a V_(z)(x_(i)) determining section for determining V_(z)(x_(i)) for each node x_(i) determined by the Gaussian quadrature operation section; a V′_(z0)(x_(i)), V_(z1)(x_(i)), and Q_(z0), Q_(z1) determining section for determining V′_(z0)(x_(i)), V_(z1)(x_(i)) for each node x_(i) determined by the Gaussian quadrature operation section, and Q_(z0) and Q_(z1) each represented by a respective one of the Formulas (19) and (20); and a Q_(N) determining section for determining an approximate value Q_(N), the approximate value Q_(N) being represented by the Formula (21).

In the apparatus using the V_(z1)(x), the Gaussian quadrature operation section is preferably configured to use the V_(z1)(x) selected to allow V′_(z0)(x) to be always zero or more in the flow passage cross-section S, or to be always zero or less in the flow passage cross-section S. However, as described above, it may also be possible to have the node x_(i) in the integral interval even if the sign of the weighting function is changed in the integral interval. Therefore, it is not essential to select V_(z1)(x) in such a manner as to allow the sign of the weighting function to be constant.

In the above apparatus provided by the subject technology, the V_(z)(x_(i)) determining section may be configured to comprise:

a pair of ultrasonic elements, each placed at a position corresponding to each node x_(i) on a flow passage pipe arrangement defining the flow passage;

an ultrasonic wave transmission and receipt circuit for giving an instruction of transmission and receipt of ultrasonic wave to the pair of ultrasonic elements;

an ultrasonic wave transmission and receipt signal recording circuit for recording an ultrasonic wave transmission and receipt-indicative signal indicative of transmission and receipt of the ultrasonic wave; and

a flow velocity calculation circuit for calculating V_(z)(x_(i)) for each node x_(i) using the ultrasonic wave transmission and receipt-indicative signal recorded in the ultrasonic wave transmission and receipt signal recording circuit.

In addition, the above apparatus provided by the subject technology may further comprise an error range estimating section for estimating an error range of the approximate value Q_(N) based on the Formula (22), using a second approximate value Q_(SUB) that is less reliable than the approximate value Q_(N).

Examples of Effect of Invention

According to one or more implementations of the subject technology, it is possible to formulate Gaussian quadrature optimal to a given flow velocity distribution. This makes it possible to accurately measure a flow rate under a small number of measurement points. The method and apparatus of one or more implementations of the subject technology are particularly effective for flow velocity distribution of unstraightened and distorted flow rate distribution.

In the Gaussian quadrature used in the subject technology, the integrand is not the flow velocity distribution itself, but represents a ratio of an actual flow velocity distribution with respect to an estimated flow velocity distribution. If a certain degree of probability of flow velocity distribution can be estimated in advance, the integrand becomes closer to a constant function even under a condition of significantly distorted flow velocity distribution and the like, so that it is possible to obtain a highly accurate approximate value of the flow rate.

Further, according to the error range estimation technique taught by the subject technology, it becomes possible to guess uncertainty of the actually measured flow rate (corresponding to an approximate error range of the integration) in a simplified manner. This makes it possible to evaluate the uncertainty of flow rate caused by an integration error without necessarily performing a comparison to a reference instrument. Of course, however, it is more secure to perform a comparison testing to the reference instrument if possible. This error range estimation technique is also applicable to a conventional art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating one example of a flow passage in which a flow rate measurement according to the subject technology may be performed.

FIG. 2 is a diagram illustrating a flow passage cross-section S₁ of the flow passage illustrated in FIG. 1.

FIG. 3 is a diagram illustrating a route A*B* along which an ultrasonic wave is propagated in the flow passage illustrated in FIG. 1.

FIG. 4 is a diagram illustrating a route A*B* along which an ultrasonic wave is propagated in the flow passage illustrated in FIG. 1, as viewed from a direction orthogonal to an axis of the flow passage.

FIG. 5 is a structural diagram illustrating one embodiment of a flow rate measurement apparatus connected to a flow passage pipe arrangement 1, according to the subject technology. Note that although the pair of ultrasonic elements 2 a, 2 b is disposed on the flow passage pipe arrangement 1 in a number corresponding to the number of measurement points x_(i), FIG. 5 depicts only one pair as a representative.

FIG. 6 is a graph illustrating an example of flow velocity distribution in a flow passage of a circular pipe.

FIG. 7 is a diagram illustrating transmission and receipt points Ai*, Bi* of the ultrasonic wave when the flow rate measurement according to the subject technology is performed with the setting of N =3. Note, however, that positions of the measurement points x₁ to x₃ are not accurate. Further, the Ai*, Bi* are shifted from the flow passage cross-section S₁ in a z-direction by a given distance as in FIG. 3, but FIG. 7 does not illustrate a z-coordinate.

FIG. 8 is a structural diagram illustrating one embodiment of a flow rate measurement apparatus connected to a flow passage pipe arrangement 1, according to the subject technology. Note that although the pair of ultrasonic elements 2 a, 2 b is disposed on the flow passage pipe arrangement 1 in a number corresponding to the number of measurement points x_(i), FIG. 8 depicts only one pair as a representative.

FIG. 9A is a diagram illustrating a flow passage of a circular pipe having a baffle plate provided therein, as an example of the flow passage pipe arrangement 1 in FIG. 8

FIG. 9B is a plane view of the baffle plate as viewed in an xy-plane.

FIG. 10 is a graph illustrating an error in a flow rate as measured in a cross-section at point A in FIG. 9 according to a conventional Gaussian quadrature (Gauss-Chebyshev type) and an error in a flow rate as measured similarly in a cross-section at point A according to a method of the subject technology.

FIG. 11 is a diagram illustrating a surface S2 inclined at an angle of φ (phi) with respect to the axis of the flow passage, as another example of the flow passage cross-section.

FIG. 12 is a diagram illustrating the flow passage cross-section S₂ as viewed from a normal direction thereof.

FIG. 13 is a diagram illustrating a method for determining a normal-directional component v_(z) of a flow velocity v when the flow passage cross-section S₂ is used.

DESCRIPTION OF EMBODIMENTS

With reference to the drawings, specific examples of a flow rate measurement method and a flow rate measurement apparatus of the subject technology will now be described. It is noted, however, that the flow rate measurement method and the flow rate measurement apparatus of the subject technology are not limited to the specific aspects described below, but are modifiable within a scope of the subject technology. For example, in the first embodiment, a flow rate of a fluid flowing through a flow passage of a circular pipe having a radius 1 is measured, but it is also possible to measure a flow rate of a fluid flowing through any other flow passage according to the subject technology. (Conventionally, the integral interval of Gaussian quadrature is deemed as [−1, 1]. Even when the x-interval of the flow passage cross-section is different therefrom, it is possible to have the integral interval of [−1, 1] by an appropriate variable transformation.) Further, in the respective embodiments below, as means for measuring an average value of normal-directional component of the flow velocity with respect to the flow passage cross-section, a system (a propagation time difference system) is used in which two ultrasonic elements are placed for every measurement point on the flow passage pipe arrangement, and a flow rate is determined from a difference in propagation times of the ultrasonic wave propagated in both directions between the ultrasonic elements. However, the embodiments are not so limited, and it is also possible to alternatively use any system such as a Pulse Doppler system suitable for opaque liquid. It is also possible to use, as a system other than those using the ultrasonic wave, any publicly known systems, and any system that will be feasible in the future. Further, in the embodiments below, the flow rate Q is measured as a volume flow rate, but alternatively, it is also possible to derive a mass flow rate by, for example, multiplying the measured volume flow rate by a density. Such a measurement of mass flow rate necessarily involves a measurement of volume flow rate, meaning that it falls within a technical scope of the subject technology. Furthermore, circuits illustrated in FIGS. 5 and 8 may be physically-isolated separate circuits, or conceptual and functional sections bearing each function in one circuit. Each circuit may comprise a plurality of circuits. For example, a V′_(z0)(x_(i)), V_(z1)(x_(i)), Q_(z0), Q_(z1) calculation circuit in FIG. 8 may be a circuit group consisting of a circuit for calculating V′_(z0)(x_(i)), a circuit for calculating V_(z1)(x_(i)), a circuit for calculating Q_(z0), and a circuit for calculating Q_(z1). Respective processing performed to implement the subject technology can be arbitrarily distributed to any one or more elements.

Gaussian Quadrature Herein

As used herein and in the Claim, the term “Gaussian quadrature” is a collective term referring to a general numerical integration method determining the finite number of nodes and coefficients by using a weighting function and performed as a polynomial approximation by using the nodes and coefficients. Specific values of the nodes and coefficients are dependent on a condition such as the weighting function. The Gaussian quadrature in such a general sense will be described below.

(1) A weighting function and an integral interval are set, respectively, to w(x) and X, and assigned as follows:

I _(k)=∫_(X) dxw(x)x ^(k)   (23).

(2) With respect to the following orthogonal polynomial:

$\begin{matrix} {{P_{N}(x)} = {\sum\limits_{k = 0}^{N}{p_{k}x^{k}}}} & (26) \end{matrix}$

having coefficients p_(k) represented by the following formula:

$\begin{matrix} {\begin{bmatrix} p_{0} \\ p_{1} \\ \vdots \\ p_{N - 1} \end{bmatrix} = {- {\begin{bmatrix} I_{0} & I_{1} & \ldots & I_{N - 1} \\ I_{1} & I_{2} & \ldots & I_{N} \\ \vdots & \vdots & \ddots & \vdots \\ I_{N - 1} & I_{N} & \ldots & I_{{2N} - 2} \end{bmatrix}^{- 1}\begin{bmatrix} I_{N} \\ I_{N + 1} \\ \vdots \\ I_{{2N} - 1} \end{bmatrix}}}} & (24) \\ {{p_{N} = 1},} & (25) \end{matrix}$

roots satisfying P_(N)(x)=0 are set as nodes x_(i) (i=1, 2, . . . , N) of Gaussian quadrature.

(3) Coefficients w_(i) of Gaussian quadrature are derived by the following formula (27), in such a manner that a correct integrated value can be obtained in the case where the integrand is a polynomial of degree less than N:

$\begin{matrix} {\begin{bmatrix} w_{1} \\ w_{2} \\ \vdots \\ w_{N} \end{bmatrix} = {{\begin{bmatrix} \left( x_{1} \right)^{0} & \left( x_{2} \right)^{0} & \ldots & \left( x_{N} \right)^{0} \\ \left( x_{1} \right)^{1} & \left( x_{2} \right)^{1} & \ldots & \left( x_{N} \right)^{1} \\ \vdots & \vdots & \ddots & \vdots \\ \left( x_{1} \right)^{N - 1} & \left( x_{2} \right)^{N - 1} & \ldots & \left( x_{N} \right)^{N - 1} \end{bmatrix}^{- 1}\begin{bmatrix} I_{0} \\ I_{1} \\ \vdots \\ I_{N - 1} \end{bmatrix}}.}} & (27) \end{matrix}$

(4) With respect to a general integrand f(x), the integrated value is approximated with a finite sum as follows:

$\begin{matrix} {{\int_{X}{{{{xw}(x)}}{f(x)}}} = {{\sum\limits_{i = 1}^{N}{w_{i}{f\left( x_{i} \right)}}} + {R_{N}.}}} & (28) \end{matrix}$

The error term R_(N) is represented by a form including a 2N-th derivative, so that, when the integrand f(x) is a polynomial of degree less than 2N, the approximation error becomes 0.

First Embodiment Flow Rate Measurement Method According to the Subject Technology

According to a flow rate measurement method in one embodiment of the subject technology, the approximate value Q_(N) shown in the above Formula (14) can be obtained through a following procedure in the above Gaussian quadrature: setting the weighting function w(x) to V_(z0)(x)L(x) (Formula (12)); determining the nodes x_(i) and the coefficients w_(i) using the Formulas (23) to (27); setting the integrand f(x) to V_(z)(x)/V_(z0)(x) (Formula (15)); and performing an arithmetic operation with the Formula (28).

Error Range Estimation According to the Subject Technology

Further, according to one embodiment of the subject technology, it is possible to estimate a range of error Q-Q_(N) as described below.

An approximate value Q_(N) of the flow rate Q that is deemed to be most probable is derived according to a flow rate measurement method in one embodiment of the subject technology. In addition, an approximate value Q_(SUB) that is less reliable than Q_(N), namely an approximate value Q_(SUB) presumed to satisfy the following formula is derived by, for example, decreasing the number of measurement points utilized, and then using the flow rate measurement method of the subject technology or any other methods:

Q−Q _(N) |<|Q−Q _(SUB)|  (29).

Considering first a case of Q_(SUB)<Q_(N), (1) When Q_(N)≦Q (Q_(N) is equal or less than Q), the following formula is derived:

Q−Q _(N)≧0, Q _(SUB) −Q _(N)<0   (30).

From this formula, the following formula is obtained:

$\begin{matrix} {{Q - Q_{N}} > {\frac{Q_{SUB} - Q_{N}}{2}.}} & (31) \end{matrix}$

(2) When Q_(N)>Q,

firstly assuming Q_(SUB)≧Q (Q_(SUB) is equal or greater than Q), Q_(SUB)<Q_(N): Q_(SUB)−Q<Q_(N)−Q. This is inconsistent with the assumption that Q_(SUB) is less reliable than Q_(N). Thus, the following relation is employed: Q_(SUB)<Q. In this case, the following formula is derived from this assumption:

Q _(N) −Q<Q−Q _(SUB)   (32).

Then, the Formula (32) is arranged by adding Q_(N) to each side thereof, to thereby obtain the Formula (31).

Considering next a case of Q_(SUB)>Q_(N),

(1) When Q_(N)≦Q (Q_(N) is equal or less than Q),

firstly, assuming Q_(SUB)≦Q (Q_(SUB) is equal or less than Q), Q_(SUB)>Q_(N): Q−Q_(SUB)<Q−Q_(N). This is inconsistent with the assumption that Q_(SUB) is less reliable than Q_(N). Thus, the following relation is employed: Q_(SUB)>Q. In this case, the following formula is derived from this assumption:

Q−Q _(N) <Q _(SUB) −Q   (33).

Then, the Formula (33) is arranged by subtracting Q_(N) from each side thereof, to thereby obtain the following Formula:

$\begin{matrix} {{Q - Q_{N}} < {\frac{Q_{SUB} - Q_{N}}{2}.}} & (34) \end{matrix}$

(2) When Q_(N)>Q, the following formula is derived:

Q−Q _(N)<0, Q _(SUB) −Q _(N)>0   (35).

From this formula, the Formula (34) is obtained.

To summarize, the error range of Q_(N) can be estimated as:

$\begin{matrix} {{{{{if}\mspace{14mu} Q_{SUB}} < Q_{N}},{\frac{Q_{SUB} - Q_{N}}{2} < {Q - Q_{N}}}}{{{{if}\mspace{14mu} Q_{SUB}} > Q_{N}},{\frac{Q_{SUB} - Q_{N}}{2} > {Q - {Q_{N}.}}}}} & (36) \end{matrix}$

One example of the error range estimation using the Formula (36) will be provided below. As an approximate value that is less reliable than the approximate value Q_(N) of the flow rate obtained by using V_(z)(x) measured at N measurement points according to the flow rate measurement method of the first embodiment, it is possible to use Q_(min) and Q_(max) which are represented as:

$\begin{matrix} {{Q_{m\; i\; n} = {\sum\limits_{i = 1}^{N}{w_{i}f_{m\; i\; n}}}}{{Q_{m\; {ax}} = {\sum\limits_{i = 1}^{N}{w_{i}f_{{ma}\; x}}}},}} & (37) \end{matrix}$

where f_(min) and f_(max) are minimum and maximum values of N integrand values V_(z)(x_(i))/V_(z0)(x_(i)) respectively, each derived for the N measurement points x_(i). Q_(min)<Q_(N)<Q_(max), and it is believed that Q_(N) is more accurate than Q_(min) and Q_(max) with practically sufficient probability. Therefore, if the Formula (36) is used, the error range of Q_(N) can be approximated by the following Formula:

$\begin{matrix} {\frac{Q_{m\; i\; n} - Q_{N}}{2} < {Q - Q_{N}} < {\frac{Q_{{ma}\; x} - Q_{N}}{2}.}} & (38) \end{matrix}$

Flow Rate Measurement Apparatus According to the Subject Technology

An example of an apparatus capable of performing the flow rate measurement method of the subject technology is illustrated in FIG. 5. The flow rate measurement apparatus in FIG. 5 comprises: a Gaussian quadrature operational circuit; at least one pair of ultrasonic elements 2 a, 2 b placed on the flow passage pipe arrangement 1, an ultrasonic wave transmission and receipt circuit; an ultrasonic wave transmission and receipt signal recording circuit; a flow velocity calculation circuit; a V_(z0)(x_(i)) calculation circuit; a Q_(N) determination circuit; and, optionally, an error range estimation circuit. A V_(z)(x_(i)) determining section is provided by the at least one pair of ultrasonic elements 2 a, 2 b , the ultrasonic wave transmission and receipt circuit, the ultrasonic wave transmission and receipt signal recording circuit and the flow velocity calculation circuit among these elements.

Into the Gaussian quadrature operational circuit, an estimate value v_(z0)(x, y) of a normal-directional component of the flow velocity at each position (x, y) on the flow passage cross-section is input as an estimated flow velocity distribution. As illustrated in FIG. 1, in the case of using the flow passage cross-section S₁ orthogonal to the axis of the flow passage pipe arrangement 1, the normal direction (z-direction) corresponds to the axial direction, so that an estimate value v_(z0)(x, y) of the axial flow velocity at each position (x, y) on the flow passage cross-section S₁ is input as the estimated flow velocity distribution. The Gaussian quadrature operational circuit formulates a weighting function w(x)=V_(z0)(x)L(x) from the input v_(z0)(x, y), and determines the node x_(i) and the coefficient w_(i) by the above described Gaussian quadrature.

The ultrasonic elements 2 a, 2 b are a pair of ultrasonic elements placed at an angle in an opposed relation on the flow passage pipe arrangement 1, as already described with reference to FIG. 4. Specifically, with respect to each of the nodes x_(i) (where i ranges from 1 to N) determined by the Gaussian quadrature operational circuit, points A, B on the flow passage cross-section each corresponding to a respective one of a maximum value and a minimum value of the y-coordinate are determined as illustrated in FIG. 2. Then, as illustrated in FIG. 3, assuming that two points each shifted from a respective one of points A, B in a z-direction by a given distance are represented, respectively, as A* and B*, the pair of ultrasonic elements 2 a, 2 b are installed at respective position capable of performing transmission and receipt of the ultrasonic wave between A* and B*. That is, N sets of pair of ultrasonic elements 2 a, 2 b are placed. Alternatively, it may also be possible to place multiple sets of pair of ultrasonic elements 2 a, 2 b on the flow passage pipe arrangement 1 in advance, and to allow only the pairs of ultrasonic elements 2 a, 2 b located at positions corresponding to the nodes x_(i) determined by the Gaussian quadrature operational circuit to be operated by the ultrasonic wave transmission and receipt circuit.

The ultrasonic wave transmission and receipt circuit is a circuit operable to transmit an ultrasonic wave transmission and receipt-instructing signal instructing a transmission and receipt of the ultrasonic wave to the ultrasonic elements 2 a, 2 b, and receive the ultrasonic signal received by the ultrasonic elements 2 a, 2 b, as well as transmit an ultrasonic wave transmission and receipt-indicative signal indicative of the transmission and receipt of the ultrasonic wave to the ultrasonic wave transmission and receipt signal recording circuit.

The ultrasonic wave transmission and receipt signal recording circuit is a circuit operable to record the ultrasonic wave transmission and receipt-indicative signal transmitted from the ultrasonic wave transmission and receipt circuit. In one example, a signal indicative of t₁, t₂ in the Formula (4) is transmitted from the ultrasonic wave transmission and receipt circuit and is recorded in the ultrasonic wave transmission and receipt signal recording circuit.

The flow velocity calculation circuit is a circuit operable to receive the ultrasonic wave transmission and receipt-indicative signal from the ultrasonic wave transmission and receipt signal recording circuit, and perform an arithmetic operation on the signal to calculate V_(z)(x_(i)) for each node x_(i). In one example, the flow velocity calculation circuit receives a signal indicative of t₁, t₂ in the Formula (4) from the ultrasonic wave transmission and receipt signal recording circuit, calculatingly determines an average of values of v_(z)(x, y) based on the Formula (4), and outputs the average value as V_(z)(x_(i)). It is noted that L and θ (theta) in the Formula (4) shall be geometrically calculated by, for example, the Gaussian quadrature calculation circuit along with the node x_(i) and the coefficient w_(i), and input to the flow velocity calculation circuit.

The V_(z0)(x_(i)) calculation circuit calculatingly determines V_(z0)(x_(i)) using the estimated flow velocity distribution for each node x_(i) determined by the Gaussian quadrature operation section. Since y_(min)(x), y_(max)(x) corresponding to each x-coordinate are determined by a shape of the flow passage cross-section, they can be preliminarily input to the V_(z0)(x_(i)) calculation circuit. The node x_(i) is input from the Gaussian quadrature operation section. The v_(z0)(x, y) may be preliminarily input, or may be input from the Gaussian quadrature operation section.

The Q_(N) determination circuit is a circuit operable to receive inputs, from each of the operation and calculation circuits, of the coefficient w_(i) determined by the Gaussian quadrature operational circuit, V_(z)(x_(i)) determined by the flow velocity calculation circuit, and V_(z0)(x_(i)) determined by the V_(z0)(x_(i)) calculation circuit, and to calculatingly determine the approximate value Q_(N) of the flow rate according to the Formula (14) using the above inputs.

It is noted that, in most cases, there is no analytical solution in the arithmetic operation performed by each circuit, including Gaussian quadrature. Thus, a numerical arithmetic operation is typically performed by a calculator etc.

The error range estimation circuit is a circuit operable to receive an input of Q_(N) from the Q_(N) determination circuit and an input of the approximate value Q_(SUB) that is less reliable than Q_(N) from the Q_(N) determination circuit or any other external circuit etc. (not illustrated) to estimate the error range according to the Formula (36).

Specific Example of the First Embodiment

Next, an operation of the flow rate measurement apparatus for obtaining an approximate value Q₃ of the flow rate according to the method of the subject technology on the assumption of N=3, in respective cases (FIG. 6) where an actual flow velocity distribution in a flow passage of a circular pipe having a radius 1 is represented by the following Formulas will be explained:

(Laminar Flow)

v _(z)(x, y)=1−(x ² +y ²)   (39)

(Exponential Law)

v _(z)(x, y)=(1−√{square root over (x ² +y ²)})^(1/n)   (40),

where n=5, 7, 9.

In this first embodiment, the flow passage cross-section S₁ illustrated in FIG. 1 etc. is used as the flow passage cross-section, and it is assumed that the estimated value v_(z0)(x, y) of the normal-directional component of the flow velocity is preliminarily given as:

v _(z0)(x, y)=(1−√{square root over (x ² +y ²)})^(1/7)   (41).

The estimated value v_(z0)(x, y) is input to the Gaussian quadrature operational circuit. The Gaussian quadrature operational circuit formulates the following weighting function from the input value v_(z0)(x, y), and determines the node x_(i) and the coefficient w_(i) according to the above described Gaussian quadrature.

$\begin{matrix} \begin{matrix} {{w(x)} = {{V_{z\; 0}(x)}{L(x)}}} \\ {= {\int_{- \sqrt{1 - x^{2}}}^{\sqrt{1 - x^{2}}}{{{y\left( {1 - \sqrt{x^{2} + y^{2}}} \right)}^{\frac{1}{7}}}.}}} \end{matrix} & (42) \end{matrix}$

In this specific example, x₁ through x₃ and w₁ through w₃ are determined because of N=3.

Corresponding to each of the nodes x₁ through x₃ determined above, the transmission and receipt points A₁* through A₃*, B₁* through B₃* of the ultrasonic wave are determined (FIG. 7). Specifically, the points A₁* through A₃* and B₁* through B₃* are determined by determining points A₁ through A₃ and B₁ through B₃ for the nodes x₁ through x₃ in the manner as illustrated in FIG. 2 (the points A, B in FIG. 2 obtained by setting the point x_(i) in FIG. 2 to a node x₁ are determined as the points A₁, B₁, the points A, B in FIG. 2 obtained by setting the point x_(i) in FIG. 2 to a node x₂ are determined as the points A₂, B₂, and the points A, B in FIG. 2 obtained by setting the point x_(i) in FIG. 2 to a node x₃ are determined as the points A₃, B₃), and shifting these points by a given distance in the z-direction (See FIG. 3. The points Ai and Bi are shifted to the opposite direction to each other.). Each of three pairs of ultrasonic elements 2 a, 2 b is placed at a position capable of performing transmission and receipt of the ultrasonic wave between each of A₁* and B₁*, A₂* and B₂*, and A₃* and B₃*. The placement of the ultrasonic elements can be performed by an optional actuator etc. (not illustrated) connected to the Gaussian quadrature operational circuit. As already described above, if multiple sets of pair of ultrasonic elements 2 a, 2 b are placed on the flow passage pipe arrangement 1 in advance, and only the pairs of ultrasonic elements 2 a, 2 b located at a position corresponding to the node x_(i) determined by the Gaussian quadrature operational circuit are operated by the ultrasonic wave transmission and receipt circuit, then the manipulation for placing the ultrasonic elements is not required.

Then, the ultrasonic wave transmission and receipt circuit transmits the ultrasonic wave transmission and receipt-instructing signal to the three pairs of ultrasonic elements 2 a, 2 b. The three pairs of ultrasonic elements 2 a, 2 b bi-directionally transmit and receive the ultrasonic wave each between A₁* and B₁*, A₂* and B₂*, and A₃* and B₃*, and the ultrasonic signal received by each ultrasonic element is transmitted to the ultrasonic wave transmission and receipt circuit. The ultrasonic wave transmission and receipt circuit measures, from the received ultrasonic signal, the propagation times t₁, t₂ for each direction in each transmission and receipt passage, and transmits the propagation times to the ultrasonic wave transmission and receipt signal recording circuit as the ultrasonic wave transmission and receipt-indicative signal.

The ultrasonic wave transmission and receipt signal recording circuit records the propagation times t₁, t₂ for each direction in each transmission and receipt passage. The flow velocity calculation circuit receives the propagation times t₁, t₂ from the ultrasonic wave transmission and receipt signal recording circuit, and calculates V_(z)(x₁) through V_(z)(x₃) for the nodes x₁ through x₃ according to the Formula (4).

And, the V_(z0)(x_(i)) calculation circuit calculatingly determines V_(z0)(x₁) through V_(z0)(x₃) for the nodes x₁ through x₃ using the estimated flow velocity distribution of the Formula (41) (In this case, y_(max)(x)=√{square root over ( )} (square root of) (1−x²), and y_(min)(x)=−√{square root over ( )} (square root of) (1−x²).). The Q_(N) determination circuit calculatingly determines the approximate value Q₃ of the flow rate according to the Formula (14) using the coefficients w₁ through w₃ determined by the Gaussian quadrature operational circuit, V_(z)(x₁) through V_(z)(x₃) determined by the flow velocity calculation circuit, and V_(z0)(x₁) through V_(z0)(x₃) determined by the V_(z0)(x_(i)) calculation circuit.

The error range estimation circuit receives an input of Q₃ from the Q_(N) determination circuit and an input of the approximate value Q_(SUB) that is less reliable than Q_(N) from the Q_(N) determination circuit or any external circuit etc., to estimate the error range according to the Formula (36). The approximate value Q_(SUB) may be an approximate value of the flow rate determined for N other than 3 using the above each circuit, or may be an approximate value of the flow rate determined according to any method different from the subject technology.

Experiment Using the Flow Rate Measurement Method According to the Subject Technology

For the operations other than the error range estimation among the above operations, a simulation according to a calculation simulation was performed. However, the measurement of the propagation times t₁, t₂ using the ultrasonic elements 2 a, 2 b was not performed, but V_(z)(x_(i)) was directly calculated using v_(z)(x, y) in the Formulas (39), (40). In addition, an approximate value of the flow rate was calculated in a similar way according to the conventional Gaussian quadrature. The calculation results thereof will be compared below.

The Table 1 shows the node x_(i) and coefficient w_(i) calculated for N=3, in various conventional Gaussian quadratures and the method of the subject technology. Note that, in the quadrature of the subject technology, the weighting function V_(z0)(x)L(x) was formulated according to the Formulas (12) and (13) using v_(z0)(x, y) in the Formula (41), and the node x_(i) and coefficient w_(i) are determined using the Formulas (23) to (27).

TABLE 1 N = 3 x_(i) w_(i) (conventional example) x₁ = 0.00000 w₁ = 0.88889 Gauss-Legendre x_(2, 3) = +/−0.77460 w₂ = w₃ = 0.55556 (conventional example) 0.00000 0.66667 Chebyshev +/−0.70711 0.66667 (conventional example) 0.00000 0.71111 Lobatto +/−0.65465 0.54444 (conventional example) 0.00000 0.78540 Gauss-Chebyshev +/−0.70711 0.39270 (subject technology) 0.00000 1.32062 optimized to exponential +/−0.68906 0.62251 law of n = 7

In Table 1, in the case of N=3, x₂ and x₃ are in an opposite relation between plus and minus (For example, x₁=0.0000, and x₂, x₃=+/−0.77460 in the conventional example of Gauss-Legendre type), and w₂=w₃ (w₁=0.88889, and w₂=w₃=0.55556 in the conventional example of Gauss-Legendre type. The same applies to other conventional examples and the subject technology). It is noted that fixed nodes at either end in the Lobatto type quadrature are not included in Table 1 because the integrand can be deemed as zero.

Using the node x_(i) and coefficient w_(i) in Table 1, the methods of each conventional example and the subject technology are used to determine the approximate value Q₃ of the flow rate. Further, an actual flow rate Q was calculated from the Formulas (39) and (40) to calculate an actual error in the approximate value Q₃ determined in each of the methods. These results are shown in Table 2.

TABLE 2 integration error (%) 100 x |Q_(N) − Q |/Q laminar exponential exponential exponential flow law n = 5 law n = 7 law n = 9 (conventional example) 0.69 1.21 1.23 1.25 Gauss-Legendre (conventional example) 3.40 1.61 1.91 2.05 Chebyshev (conventional example) 0.29 0.75 1.14 1.37 Lobatto (conventional example) 0.0  0.76 0.58 0.47 Gauss-Chebyshev (subject technology) 0.09 0.23 0.00 0.14 optimized to exponential law of n = 7

Comparing the theoretical values of integration errors for the conventional techniques and for the technique of the subject technology, the technique of the subject technology is more advantageous in most cases. In particular, where the flow velocity distribution is closer to v_(z0)(x, y), the effect is larger.

Second Embodiment

As already described above, in the case where the sign of the weighting function is not constant due to a reverse flow etc., it is not always possible to have the node x_(i) of the Gaussian quadrature in the integral interval. A method of virtually offsetting the zero-point of the flow velocity distribution in this case to apparently avoid the reverse flow will be described below.

This method is based on an idea of using an appropriate function V_(z1)(x) to offset the zero-points of V_(z0)(x), V_(z)(x) as the following Formulas (43) and (44), and selecting V_(z1)(x) particularly in such a manner as to allow V′_(z0)(x) to be always zero or more in the flow passage cross-section S, or to be always zero or less in the flow passage cross-section S, thereby to keep the sign of the weighting function to be constant:

V′ _(z0)(x)=V _(z0)(x)+V _(z1)(x)   (43)

$\begin{matrix} {{V_{z}^{\prime}(x)} = {{V_{z}(x)} + {\frac{Q_{N}}{Q_{z\; 0}}{{V_{z\; 1}(x)}.}}}} & (44) \end{matrix}$

In this case, the following formula is applied.

Q _(z0)=∫_(x) _(min) ^(x) ^(max) dxV _(z0)(x)L(x)

Q _(z1)=∫_(x) _(min) ^(x) ^(max) dxV _(z1)(x)L(x)   (45).

Using the V′_(z0)(x), V′_(z)(x) obtained based on the offset zero-point, it is possible to formulate a flow rate Q′ (46) obtained based on the offset zero-point, an integrand (47) for applying Gaussian quadrature to the flow rate Q′, and a weighting function (48) as follows:

$\begin{matrix} {{Q^{\prime} = {\int_{x_{m\; i\; n}}^{x_{{ma}\; x}}{{{{xV}_{z}^{\prime}(x)}}{L(x)}}}},} & (46) \\ {\frac{V_{z}^{\prime}(x)}{V_{z\; 0}^{\prime}(x)},} & (47) \\ {{V_{z\; 0}^{\prime}(x)}{{L(x)}.}} & (48) \end{matrix}$

If the weighting function of the Formula (48) is used to determine the node x₁ and coefficient w_(i) of Gaussian quadrature, an approximate value Q′_(N) of the flow rate Q′ can be expressed as follows by using the x_(i) and w_(i).

$\begin{matrix} {Q_{N}^{\prime} = {\sum\limits_{i = 1}^{N}{w_{i}{\frac{V_{z}^{\prime}\left( x_{i} \right)}{V_{\; {z\; 0}}^{\prime}\left( x_{i} \right)}.}}}} & (49) \end{matrix}$

Further, from the Formulas (44) to (46), the following formula is obtained:

$\begin{matrix} {Q = {Q^{\prime} - {\frac{Q_{z\; 1}}{Q_{z\; 0}}{Q_{N}.}}}} & (50) \end{matrix}$

Thus, by replacing Q and Q′ with approximate values Q_(N) and Q′_(N) respectively, the following formula is obtained:

$\begin{matrix} {Q_{N} = {Q_{N}^{\prime} - {\frac{Q_{z\; 1}}{Q_{z\; 0}}Q_{N}}}} & (51) \end{matrix}$

Furthermore, from the Formulas (44), (49) and (51), the following formula is obtained:

$\begin{matrix} {Q_{N} = {\frac{\sum\limits_{i = 1}^{N}{w_{i}\frac{V_{z}\left( x_{i} \right)}{V_{z\; 0}^{\prime}\left( x_{i} \right)}}}{1 + \frac{Q_{z\; 1}}{Q_{z\; 0}} - {\frac{1}{Q_{z\; 0}}{\sum\limits_{i = 1}^{N}{w_{i}\frac{V_{z\; 1}\left( x_{i} \right)}{V_{z\; 0}^{\prime}\left( x_{i} \right)}}}}}.}} & (52) \end{matrix}$

According to the Formula (52), it becomes possible to determine the approximate value Q_(N) of the flow rate even in the presence of reverse flow and the like. Specifically, the approximate value Q_(N) can be determined by: determining the node x_(i) and coefficient w_(i) of Gaussian quadrature using the weighting function V′_(z0)(x)L(x) obtained based on an offset zero-point; determining V_(z)(x_(i)) through the ultrasonic wave measurement or the like as described above for the determined measurement points x_(i); calculatingly determining each of V′_(z0)(x_(i)), V_(z1)(x_(i)), Q_(z0) and Q_(z1) according to the definitional formulas; and substituting these values into the Formula (52).

In this case, as already described above, it may also be possible to have the node x_(i) in the integral interval even if the sign of the weighting function is changed in the integral interval. For example, when the estimated flow velocity distribution is given by the following Formula (53), the sign of V_(z0)(x) is changed in the interval of [−1, 1] in the flow passage of a circular pipe having a radius 1 of −1≦x, y≦1 (x,y are equal or greater than −1, and equal or less than 1). However, application of Gaussian quadrature of the Formulas (23) to (27), where the weighting function is set to V_(z0)(x)L(x) (N=3), results in nodes x₁, x₂, x₃=0.00, 0.75, −0.75, and coefficients w_(i), w₂, w₃=0.05, −0.36, −0.36, so that it becomes possible to have the node x_(i) in the integral interval:

$\begin{matrix} {{v_{z\; 0}\left( {x,y} \right)} = {\cos \; \frac{3}{2}{{\pi \left( {x^{2} + y^{2}} \right)}.}}} & (53) \end{matrix}$

In this case, V_(z0)(x) corresponding to x₁ to x₃ is V_(z0)(x_(i))=0.19, −0.81, −0.81, so that the sign of the weighting function is surely changed. Also, in the case of introducing the offset function V_(z1)(x), if the estimated flow velocity distribution is represented as the following Formula (54), and the offset function as V_(z1)(x)=1, for example, then V′_(z0)(x) obtained by using the V_(z1)(x) and v_(z0)(x, y) in the Formula (54) is the same as V_(z0)(x) obtained according to the Formula (13) using the Formula (53) (when the offset is not introduced). Thus, when Gaussian quadrature is performed with weighting function being V′_(z0)(x)L(x), the node x_(i) (x₁, x₂, x₃=0.00, 0.75, −0.75) is obtained in the integral interval:

$\begin{matrix} {{v_{z\; 0}\left( {x,y} \right)} = {{\cos \; \frac{3}{2}{\pi \left( {x^{2} + y^{2}} \right)}} - 1.}} & (54) \end{matrix}$

Therefore, in both of the first and second embodiments, even if the sign of the weighting function is changed in the integral interval, it may be possible to obtain the approximate value of the flow rate according to Gaussian quadrature, theoretically.

For the approximate value Q_(N) obtained using the Formula (52), it is also possible to estimate the error range according to the Formula (36) using the approximate value Q_(SUB) that is less reliable than the Q_(N).

Flow Rate Measurement Apparatus According to the Subject Technology

One example of an apparatus capable of performing the flow rate measurement method of the subject technology described in the second embodiment is illustrated in FIG. 8. In the apparatus in FIG. 8, a V′_(z0)(x), V_(z1)(x_(i)), Q_(z0), Q_(z1) calculation circuit is used as substitute for the V_(z0)(x) calculation circuit in the apparatus illustrated in FIG. 5. The structure and operation of the apparatus in FIG. 8 will be described below, in which an explanation of the same structure and operation as in the apparatus in FIG. 5 will be omitted.

The flow rate measurement apparatus in FIG. 8 comprises: a Gaussian quadrature operational circuit; at least one pair of ultrasonic elements 2 a, 2 b placed on the flow passage pipe arrangement 1, an ultrasonic wave transmission and receipt circuit; an ultrasonic wave transmission and receipt signal recording circuit; a flow velocity calculation circuit; a V′_(z0)(x_(i)), V_(z1)(x_(i)), Q_(z0), Q_(z1) calculation circuit; a Q_(N) determination circuit, and, optionally, an error range estimation circuit. A V_(z)(x_(i)) determining section is provided by the at least one pair of ultrasonic elements 2 a, 2 b, the ultrasonic wave transmission and receipt circuit, the ultrasonic wave transmission and receipt signal recording circuit and the flow velocity calculation circuit among these elements.

Into the Gaussian quadrature operational circuit, an estimate value v_(z0)(x, y) of a normal-directional component of the flow velocity at each position (x, y) in the flow passage cross-section is input as an estimated flow velocity distribution, and preferably (although not necessary), an offset function V_(z1)(x) is input which is selected in such a manner as to allow V′_(z0)(x) represented by the Formula (16) to be always zero or more in the flow passage cross-section S, or to be always zero or less in the flow passage cross-section S. The Gaussian quadrature operational circuit formulates a weighting function w(x)=V′_(z0)(x)L(x) from the input v_(z0)(x, y), V_(z1)(x), and determines the node x_(i) and the coefficient w_(i) according to the above described Gaussian quadrature.

The ultrasonic elements 2 a, 2 b may be the same elements as described in relation to the apparatus in FIG. 5. The measurement point x_(i) at which the ultrasonic elements are to be disposed is determined using the modified weighting function w(x)=V′_(z0)(x)L(x). Each of the ultrasonic wave transmission and receipt circuit, the ultrasonic wave transmission and receipt signal recording circuit and the flow velocity calculation circuit may also be the same circuit as described in relation to the apparatus in FIG. 5.

The V′_(z0)(x_(i)), V_(z1)(x_(i)), Q_(z0), Q_(z1) calculation circuit is a circuit operable to calculate V′_(z0)(x_(i)) and V_(z1)(x_(i)) for each node x_(i) determined by the Gaussian quadrature operation section and calculatingly determine Q_(z0) and Q_(z1). The node x_(i) is input from the Gaussian quadrature operation section. The v_(z0)(x, y) may be preliminarily input, or may be input from the Gaussian quadrature operation section.

The Q_(N) determination circuit is a circuit operable to receive inputs, from each of the operation and calculation circuits, of the coefficient w_(i) determined by the Gaussian quadrature operational circuit, V_(z)(x_(i)) determined by the flow velocity calculation circuit, and V′_(z0)(x_(i)), V_(z1)(x_(i)), Q_(z0) and Q_(z1) determined by the V′_(z0)(x_(i)), V_(z1)(x_(i)), Q_(z0), Q_(z1) calculation circuit, and calculatingly determine the approximate value Q_(N) of the flow rate according to the Formula (21) using the inputs. As with the first embodiment, the arithmetic operation performed by each circuit is typically performed using a calculator etc.

The error range estimation circuit is a circuit operable to receive an input of Q_(N) from the Q_(N) determination circuit and an input of the approximate value Q_(SUB) that is less reliable than Q_(N) from the Q_(N) determination circuit or any other external circuit etc. to estimate the error range according to the Formula (36).

Operation of the Flow Rate Measurement Apparatus in the Second Embodiment

Next, an operation of the flow rate measurement apparatus for obtaining an approximate value Q₃ of the flow rate according to the method of the subject technology on the assumption of N=3 will be explained, in the case where the flow velocity distribution is represented by v_(z)(x, y). However, the explanation of the same operation as that described in the first embodiment will be omitted.

In the second embodiment, the flow passage cross-section S₁ illustrated in FIG. 1 etc. is used as a flow passage cross-section, and the estimate value of the normal-directional component of the flow velocity is assumed to be v_(z0)(x, y). The estimate value v_(z0)(x, y) is input to the Gaussian quadrature operational circuit. Further, as V_(z1)(x), preferably (although not necessary), the V_(z1)(x) selected to allow V′_(z0)(x)=V_(z0)(x)+V_(z1)(x) to be always zero or more, or to be always zero or less in the flow passage cross-section S₁ is also input to the Gaussian quadrature operational circuit.

The Gaussian quadrature operational circuit formulates a weighting function w(x)=V′_(z0)(x)L(x) from the input v_(z0)(x, V_(z1)(x), and determines the node x_(i) and the coefficient w_(i) according to the above described Gaussian quadrature. In this specific example, x₁ through x₃ and w₁ through w₃ are determined because of N=3.

Corresponding to each of the nodes x₁ through x₃ determined above, the transmission and receipt points A₁* through A₃*, B₁* through B₃* of the ultrasonic wave are determined (FIG. 7. However, an actual value of each of x₁ through x₃ is different from that in the first embodiment). Specifically, the points A₁* through A₃* and B₁* through B₃* are determined by determining points A₁ through A₃ and B₁ through B₃ for the nodes x₁ through x₃ in the manner as illustrated in FIG. 2 (the points A, B in FIG. 2 obtained by setting the point x_(i) to a node x₁ in FIG. 2 are determined as the points A₁, B₁, the points A, B in FIG. 2 obtained by setting the point x_(i) to a node x₂ in FIG. 2 are determined as the points A₂, B₂, and the points A, B in FIG. 2 obtained by setting the point x_(i) to a node x₃ in FIG. 2 are determined as the points A₃, B₃), and shifting these points by a given distance in the z-direction (See FIG. 3. The points Ai and Bi are shifted to the opposite direction to each other.). Each of three pairs of ultrasonic elements 2 a, 2 b is placed at a position capable of performing transmission and receipt of the ultrasonic wave between each of A₁* and B₁*, A₂* and B₂*, and A₃* and B₃*.

Then, the ultrasonic wave transmission and receipt circuit transmits the ultrasonic wave transmission and receipt-instructing signal to the three pairs of ultrasonic elements 2 a, 2 b. The three pairs of ultrasonic elements 2 a, 2 b bidirectionally transmit and receive the ultrasonic wave each between A₁* and B₁*, A₂* and B₂*, and A₃* and B₃*, and the ultrasonic signal received by each ultrasonic element is transmitted to the ultrasonic wave transmission and receipt circuit. The ultrasonic wave transmission and receipt circuit measures, from the received ultrasonic signal, the propagation times t₁, t₂ for each direction in each transmission and receipt passage, and transmits the propagation times to the ultrasonic wave transmission and receipt signal recording circuit as the ultrasonic wave transmission and receipt-indicative signal.

The ultrasonic wave transmission and receipt signal recording circuit records the propagation times t₁, t₂ for each direction in each transmission and receipt passage. The flow velocity calculation circuit receives the propagation times t₁, t₂ from the ultrasonic wave transmission and receipt signal recording circuit, and calculates V_(z)(x₁) through V_(z)(x₃) for the nodes x₁ through x₃ according to the Formula (4).

In addition, the V′_(z0)(x_(i)), V_(z1)(x_(i)), Q_(z0), Q_(z1) calculation circuit calculates V′_(z0)(x_(i)) and V_(z1)(x_(i)) for each of the nodes x₁ through x₃ and calculatingly determines Q_(z0) and Q_(z1). The Q_(N) determination circuit calculatingly determines the approximate value Q₃ of the flow rate according to the Formula (21) using the coefficients w₁ through w₃ determined by the Gaussian quadrature operational circuit, V_(z)(x₁) through V_(z)(x₃) determined by the flow velocity calculation circuit, and V′_(z0)(x₁) through V′_(z0)(x₃), V_(z1)(x₁) through V_(z1)(x₃), Q_(z0) and Q_(z1) determined by the V′_(z0)(x_(i)), V_(z1)(x_(i)), Q_(z0), Q_(z1) calculation circuit.

The error range estimation circuit receives an input of value of Q₃ from the Q_(N) determination circuit and an input of the approximate value Q_(SUB) that is less reliable than Q_(N) from the Q_(N) determination circuit or any external circuit etc., to estimate the error range according to the Formula (36). The approximate value Q_(SUB) may be an approximate value of the flow rate determined for N other than 3 using the above each circuit, or may be an approximate value of the flow rate determined according to any method different from the subject technology.

Experiment Using the Flow Rate Measurement Method According to the Subject Technology

In a practical fluid facility, the flow velocity distribution may be disturbed by various structural instruments and objects in the flow passage, resulting in errors occurring in the flow rate measurement. For the flow rate measurement accuracy in such a flow passage having unstraightened flow, effectiveness of the subject technology will be exemplified by comparing the result of measurement using the flow rate measurement method of the subject technology with the result of measurement according to the conventional art.

(Implementation Method)

As a typical example of a passage of unstraightened flow, consideration will be made on a flow passage of a circular pipe having a bent tube (elbow portion) and a baffle plate (partially sealing the flow passage cross-section downstream the bent tube) disposed therein (FIG. 9A, 9B). The baffle plate is disposed at a position X in FIG. 9A, and a cross-section thereof is semicircular as illustrated in FIG. 9B. In this regard, the flow passage cross-section has a circular shape having a diameter of 83 mm.

Into the flow passage of a circular pipe in FIG. 9A, a test fluid (water) continues to flow with a constant flow rate of 5 (liter/sec.). According to the second embodiment, a flow rate is measured at a cross-section A (immediately downstream the bent tube and baffle plate: a position about 200 mm away from the position X in an axial direction). Note that the estimated flow velocity distribution is derived by a preliminary simulation. A reverse flow occurs downstream the baffle plate, so that it is required to formulate Gaussian quadrature with virtually offsetting the zero-point of the flow velocity distribution. In this case, by taking V_(z1)(x) as a constant function (single the flow passage cross-section average of the estimated flow velocity), the weighting function was formulated based on the Formulas (16) to (18), and the flow rate was measured according to the method of the second embodiment.

Further, as a comparative target, a flow rate was measured at the cross-section A according to the conventional art in the Patent Documents 3 and 4 (Gauss-Chebyshev type quadrature). Furthermore, as a reference value of the flow rate, a flow rate was also measured at the cross-section B (sufficiently downstream the bent tube and baffle plate: a position of 3000 mm or more away from the position X in an axial direction) using a general-purpose flow rate measurement apparatus. Each value of the node x_(i) and a coefficient w_(i) of Gaussian quadrature, V_(z0)(x_(i)), Q_(z0), V_(z1)(x_(i)) and Q_(z1) when the flow rate is measured according to the method of the second embodiment of the subject technology is as follows. Note that the radius of the flow passage is assumed to be 1, and the estimated flow velocity is normalized to be 1 in an average of flow passage cross-section. When the flow rate was measured according to the Gauss-Chebyshev type quadrature (N=3), the node x_(i) was: x₁, x₂, x₃=0.00, 0.71, −0.71, and the coefficient w_(i) was: w₁, w₂, w₃=0.79, 0.39, 0.39.

TABLE 3 N = 3 x_(i) w_(i) V_(z0)(x_(i)) Q_(z0) (subject technology) i = 1 0.12 3.20 1.19 3.14 second embodiment i = 2 0.73 2.17 2.33 i = 3 −0.63 0.93 −0.10 N = 3 V_(z1)(x_(i)) Q_(z1) error (%) (subject technology) i = 1 1.00 3.14 0.20 second embodiment i = 2 1.00 i = 3 1.00

The errors in the flow rates each measured at the cross-section A according to the method of the second embodiment and the conventional art were evaluated by comparing with the reference flow rate value measured at the cross-section B.

FIG. 10 illustrates an absolute value of a difference between “a flow rate measured at a cross-section B” and “a flow rate measured at a cross-section A” in each case of using a conventional Gauss-Chebyshev quadrature and of using the method of the second embodiment, as a flow rate measurement method at the cross-section A. The figure shows that the subject technology is capable of reducing the flow rate measurement error to about the one-tenth thereof, as compared to the conventional art.

As already described above, even when the flow passage cross-section is an inclined or curved surface, the subject technology makes it possible to measure the flow rate in the same principle. For example, when a flow passage cross-section S₂ inclined at an angle φ (phi) with respect to an axis of the flow passage (which is obtained by rotating the flow passage cross-section S₁ by φ (phi) about the x-axis in FIG. 1) is used as illustrated in FIG. 11, the flow rate can be measured by defining the x- and y-coordinates on the flow passage cross-section S₂ as illustrated in FIG. 12, and in the similar method as that already described above. In this case, if the fluid is flowing in the axial direction of the flow passage pipe arrangement 1 as illustrated in FIG. 11, the normal-directional (z-directional) component of the flow velocity with respect to the flow passage cross-section S₂ is represented by v_(z)(x, y)=v(x, y)·cos φ (phi), where v(x, y) is a measure of the flow velocity (FIG. 13). Further, each of the positions A*, B* for performing transmission and receipt of the ultrasonic wave may be the position shifted from each of A, B illustrated in FIG. 12 by a given distance in axially opposite directions of the flow passage pipe arrangement 1. In this case, in the above-described propagation time difference system, the average flow velocity obtained by using the Formula (4) multiplied by cos φ (phi) provides V_(z)(x_(i)). Even in the case where the flow passage cross-section is a curved surface, v_(z)(x, y) can be determined to perform the flow rate measurement in a similar manner by using a normal-directional component corresponding to each position (x, y) on the flow passage cross-section (using different φ (phi) at each position (x, y)). Alternatively, it is also possible to determine a plane which is best approximated to this curved surface by means of, for example, a least-square approach, and, for example, use the determined plane as a virtual flow passage cross-section, thereby to measure the average flow velocity using the Formula (4) (set a representative value of the inclined angle to φ (phi), and multiply by cos φ (phi)) and determine the flow rate.

EXAMPLES OF INDUSTRIAL APPLICABILITY

The measurement method and measurement apparatus of the subject technology can be potentially used for high-accuracy general flow rate measurement. For example, when used in an actual plant etc., it becomes possible to perform an accurate measurement under the flow velocity distribution of unstraightened flow in contrast to the conventional way. Thus, it can be expected, for example, to shorten the pipe line elongated for flow-straightening, improve the degree of freedom in a position for attaching a flowmeter, and so on.

EXPLANATION OF CODES

-   1: flow passage pipe arrangement -   2 a, 2 b: ultrasonic element 

1. A method for measuring a flow rate Q of a fluid flowing through a flow passage, the flow rate Q being represented by the following Formula: $\begin{matrix} \begin{matrix} {Q = {\int_{S}{{sv}_{z}}}} \\ {= {\int_{x_{m\; i\; n}}^{x_{m\; {ax}}}{{x}{\int_{y_{m\; i\; n}{(x)}}^{y_{{ma}\; x}{(x)}}{{{yv}_{z}\left( {x,y} \right)}}}}}} \\ {= {\int_{x_{m\; i\; n}}^{x_{m\; {ax}}}{{{{xV}_{z}(x)}}{L(x)}}}} \end{matrix} & \; \\ {{{where},{{V_{z}(x)} = \frac{\int_{y_{m\; i\; n}{(x)}}^{y_{{ma}\; x}{(x)}}{{{yv}_{z}\left( {x,y} \right)}}}{{y_{{ma}\; x}(x)} - {y_{m\; i\; n}(x)}}}}{{L(x)} = {{y_{{ma}\; x}(x)} - {y_{m\; i\; n}(x)}}}} & \; \end{matrix}$ (where: S represents a flow passage cross-section, wherein a position within the flow passage cross-section S is represented as (x, y); v_(z) represents a normal-directional (z-directional) component with respect to the flow passage cross-section S of a flow velocity of the fluid, wherein v_(z)(x, y) represents a normal-directional component at a position (x, y) of a flow velocity of the fluid at the position (x, y) on the flow passage cross-section S; and y_(min)(x) and y_(max)(x) represent, respectively, a minimum value and a maximum value of the y-coordinate when the x-coordinate is fixed on the flow passage cross-section S, wherein x_(min) and x_(max) represent, respectively, a minimum value and a maximum value of the x-coordinate on the flow passage cross-section S), the method comprising: determining a node x_(i) and a coefficient w_(i) for Gaussian quadrature (where i represents a natural number of 1 to N, where N represents an arbitrary natural number equal to or greater than 1), using a weighting function V_(z0)(x) L(x) represented by, V_(z0)(x)L(x) where, ${V_{z\; 0}(x)} = \frac{\int_{y_{m\; i\; n}{(x)}}^{y_{{ma}\; x}{(x)}}{{{yv}_{z\; 0}\left( {x,y} \right)}}}{{y_{{ma}\; x}(x)} - {y_{m\; i\; n}(x)}}$ where v_(z0)(x, y) represents an estimate value of a normal-directional component of a flow velocity at each position (x, y) within the flow passage cross-section S; determining V_(z)(x_(i)) and V_(z0)(x_(i)) for respective nodes x_(i); and determining an approximate value Q_(N) of the flow rate Q, the approximate value Q_(N) being represented by using the following Formula: $Q_{N} = {\sum\limits_{i = 1}^{N}{w_{i}{\frac{V_{z}\left( x_{i} \right)}{V_{z\; 0}\left( x_{i} \right)}.}}}$
 2. A method for measuring a flow rate Q of a fluid flowing through a flow passage, the flow rate Q being represented by the following Formula: $\begin{matrix} \begin{matrix} {Q = {\int_{S}{{sv}_{z}}}} \\ {= {\int_{x_{m\; i\; n}}^{x_{m\; {ax}}}{{x}{\int_{y_{m\; i\; n}{(x)}}^{y_{{ma}\; x}{(x)}}{{{yv}_{z}\left( {x,y} \right)}}}}}} \\ {= {\int_{x_{m\; i\; n}}^{x_{m\; {ax}}}{{{{xV}_{z}(x)}}{L(x)}}}} \end{matrix} & \; \\ {{{where},{{V_{z}(x)} = \frac{\int_{y_{m\; i\; n}{(x)}}^{y_{{ma}\; x}{(x)}}{{{yv}_{z}\left( {x,y} \right)}}}{{y_{{ma}\; x}(x)} - {y_{m\; i\; n}(x)}}}}{{L(x)} = {{y_{{ma}\; x}(x)} - {y_{m\; i\; n}(x)}}}} & \; \end{matrix}$ (where: S represents a flow passage cross-section, wherein a position within the flow passage cross-section S is represented as (x, y); v_(z) represents a normal-directional (z-directional) component with respect to the flow passage cross-section S of a flow velocity of the fluid, wherein v_(z)(x, y) represents a normal-directional component at a position (x, y) of a flow velocity of the fluid at the position (x, y) on the flow passage cross-section S; and y_(min)(x) and y_(max)(x) represent, respectively, a minimum value and a maximum value of the y-coordinate when the x-coordinate is fixed on the flow passage cross-section S, wherein x_(min) and x_(max) represent, respectively, a minimum value and a maximum value of the x-coordinate on the flow passage cross-section S), the method comprising: determining a node x_(i) and a coefficient w_(i) for Gaussian quadrature (where i represents a natural number of 1 to N, where N represents an arbitrary natural number equal to or greater than 1), using a weighting function V′_(z0)(x) L(x) represented by, V′_(z0)(x)L(x) where V′ _(z0)(x)=V _(z0)(x)+V _(z1)(x) where V_(z1)(x) is an offset function, where ${V_{z\; 0}(x)} = \frac{\int_{y_{m\; i\; n}{(x)}}^{y_{{ma}\; x}{(x)}}{{{yv}_{z\; 0}\left( {x,y} \right)}}}{{y_{{ma}\; x}(x)} - {y_{m\; i\; n}(x)}}$ where v_(z0)(x, y) represents an estimate value of a normal-directional component of a flow velocity at each position (x, y) within the flow passage cross-section S; for respective nodes x_(i), determining V_(z)(x_(i)), and determining V′_(z0)(x_(i)), V_(z1)(x_(i)), and Q_(z0) and Q_(z1) each represented by a respective one of the following Formulas: Q _(z0)=∫_(x) _(min) ^(x) ^(max) dxV _(z0)(x)L(x) Q _(z1)=∫_(x) _(min) ^(x) ^(max) dxV _(z1)(x)L(x), and determining an approximate value Q_(N), the approximate value Q_(N) being represented by the following Formula: $Q_{N} = {\frac{\sum\limits_{i = 1}^{N}{w_{i}\frac{V_{z}\left( x_{i} \right)}{V_{z\; 0}^{\prime}\left( x_{i} \right)}}}{1 + \frac{Q_{z\; 1}}{Q_{z\; 0}} - {\frac{1}{Q_{z\; 0}}{\sum\limits_{i = 1}^{N}{w_{i}\frac{V_{z\; 1}\left( x_{i} \right)}{V_{z\; 0}^{\prime}\left( x_{i} \right)}}}}}.}$
 3. The method as defined in claim 2, wherein the V_(z1)(x) is selected to allow V′_(z0)(x) to be always zero or more on the flow passage cross-section S, or to be always zero or less on the flow passage cross-section S.
 4. The method as defined in claim 1, further comprising: estimating an error range of the approximate value Q_(N) based on the following Formula using a second approximate value Q_(SUB) that is less reliable than the approximate value Q_(N): ${{{if}\mspace{14mu} Q_{SUB}} < Q_{N}},{\frac{Q_{SUB} - Q_{N}}{2} < {Q - Q_{N}}}$ ${{{if}\mspace{14mu} Q_{SUB}} > Q_{N}},{\frac{Q_{SUB} - Q_{N}}{2} > {Q - {Q_{N}.}}}$
 5. The method as defined in claim 2, further comprising: estimating an error range of the approximate value Q_(N) based on the following Formula using a second approximate value Q_(SUB) that is less reliable than the approximate value Q_(N): ${{{if}\mspace{14mu} Q_{SUB}} < Q_{N}},{\frac{Q_{SUB} - Q_{N}}{2} < {Q - Q_{N}}}$ ${{{if}\mspace{14mu} Q_{SUB}} > Q_{N}},{\frac{Q_{SUB} - Q_{N}}{2} > {Q - {Q_{N}.}}}$
 6. An apparatus for measuring a flow rate Q of a fluid flowing through a flow passage, the flow rate Q being represented by the following Formula: $\begin{matrix} {Q = {\int_{S}\ {{s}\; v_{z}}}} \\ {= {\int_{x_{\min}}^{x_{\max}}\ {{x}{\int_{y_{\min}{(x)}}^{y_{\max}{(x)}}\ {{y}\; {v_{z}\left( {x,y} \right)}}}}}} \\ {= {\int_{x_{\min}}^{x_{\max}}\ {{x}\; {V_{z}(x)}{L(x)}}}} \end{matrix}$ ${where},{{V_{z\;}(x)} = \frac{\int_{y_{\min}{(x)}}^{y_{\max}{(x)}}\ {{y}\; {v_{z\;}\left( {x,y} \right)}}}{{y_{\max}(x)} - {y_{\min}(x)}}}$ L(x) = y_(max)(x) − y_(min)(x) (where: S represents a flow passage cross-section, wherein a position within the flow passage cross-section S is represented as (x, y); v_(z) represents a normal-directional (z-directional) component with respect to the flow passage cross-section S of a flow velocity of the fluid, wherein v_(z)(x, y) represents a normal-directional component at a position (x,y) of a flow velocity of the fluid at the position (x, y) on the flow passage cross-section S; and y_(min)(x) and y_(max)(x) represent, respectively, a minimum value and a maximum value of the y-coordinate when the x-coordinate is fixed on the flow passage cross-section S, wherein x_(min) and x_(max) represent, respectively, a minimum value and a maximum value of the x-coordinate on the flow passage cross-section S), the apparatus comprising: a Gaussian quadrature operation section configured to determine a node x_(i) and a coefficient w_(i) for Gaussian quadrature (where i represents a natural number of 1 to N, where N represents an arbitrary natural number equal to or greater than 1), using a weighting function V_(z0)(x) L(x) represented by, V_(z0)(x)L(x) where, ${V_{z\; 0}(x)} = \frac{\int_{y_{\min}{(x)}}^{y_{\max}{(x)}}\ {{y}\; {v_{z\; 0}\left( {x,y} \right)}}}{{y_{\max}(x)} - {y_{\min}(x)}}$ where v_(z0)(x, y) represents an estimate value of a normal-directional component of a flow velocity at each position (x, y) within the flow passage cross-section S; a V_(z)(x_(i)) determining section configured to determine V_(z)(x_(i)) for respective nodes x_(i) determined by the Gaussian quadrature operation section; a V_(z0)(x_(i)) determining section configured to determine V_(z0)(x_(i)) for respective nodes x_(i) determined by the Gaussian quadrature operation section; and a Q_(N) determining section configured to determine an approximate value Q_(N) of the flow rate Q, the approximate value Q_(N) being represented by using the following Formula: $Q_{N} = {\sum\limits_{i = 1}^{N}{w_{i}{\frac{V_{z}\left( x_{i} \right)}{V_{z\; 0}\left( x_{i} \right)}.}}}$
 7. An apparatus for measuring a flow rate Q of a fluid flowing through a flow passage, the flow rate Q being represented by the following Formula: $\begin{matrix} {Q = {\int_{S}\ {{s}\; v_{z}}}} \\ {= {\int_{x_{\min}}^{x_{\max}}\ {{x}{\int_{y_{\min}{(x)}}^{y_{\max}{(x)}}\ {{y}\; {v_{z}\left( {x,y} \right)}}}}}} \\ {= {\int_{x_{\min}}^{x_{\max}}\ {{x}\; {V_{z}(x)}{L(x)}}}} \end{matrix}$ ${where},{{V_{z\;}(x)} = \frac{\int_{y_{\min}{(x)}}^{y_{\max}{(x)}}\ {{y}\; {v_{z\;}\left( {x,y} \right)}}}{{y_{\max}(x)} - {y_{\min}(x)}}}$ L(x) = y_(max)(x) − y_(min)(x) (where: S represents a flow passage cross-section, wherein a position within the flow passage cross-section S is represented as (x, y); v_(z) represents a normal-directional (z-directional) component with respect to the flow passage cross-section S of a flow velocity of the fluid, wherein v_(z)(x, y) represents a normal-directional component at a position (x, y) of a flow velocity of the fluid at the position (x, y) on the flow passage cross-section S; and y_(min)(x) and y_(max)(x) represent, respectively, a minimum value and a maximum value of the y-coordinate when the x-coordinate is fixed on the flow passage cross-section S, wherein x_(min) and x_(max) represent, respectively, a minimum value and a maximum value of the x-coordinate on the flow passage cross-section S), the apparatus comprising: a Gaussian quadrature operation section configured to determine a node x_(i) and a coefficient w_(i) for Gaussian quadrature (where i represents a natural number of 1 to N, where N represents an arbitrary natural number equal to or greater than 1), using a weighting function V′_(z0)(x) L(x) represented by, V_(z0)(x)L(x) where V′ _(z0)(x)=V _(z0)(x)+V _(z1)(x) where V_(z1)(x) is an offset function, where ${V_{z\; 0}(x)} = \frac{\int_{y_{\min}{(x)}}^{y_{\max}{(x)}}\ {{y}\; {v_{z\; 0}\left( {x,y} \right)}}}{{y_{\max}(x)} - {y_{\min}(x)}}$ where v_(z0)(x, y) represents an estimate value of a normal-directional component of a flow velocity at each position (x, y) within the flow passage cross-section S; a V_(z)(x_(i)) determining section configured to determine V_(z)(x_(i)) for each node x_(i) determined by the Gaussian quadrature operation section; a V′_(z0)(x_(i)), V_(z1)(x_(i)), and Q_(z0), Q_(z1) determining section configured to determine V′_(z0)(x_(i)), V_(z1)(x_(i)) for respective nodes x_(i) determined by the Gaussian quadrature operation section, and Q_(z0) and Q_(z1) each represented by a respective one of the following Formulas: Q _(z0)=∫_(x) _(min) ^(x) ^(max) dxV _(z0)(x)L(x) Q _(z1)=∫_(x) _(min) ^(x) ^(max) dxV _(z1)(x)L(x), and a Q_(N) determining section configured to determine an approximate value Q_(N) of the flow rate Q, the approximate value Q_(N) being represented by the following Formula: $Q_{N} = {\frac{\sum\limits_{i = 1}^{N}{w_{i}\frac{V_{z}\left( x_{i} \right)}{V_{z\; 0}^{\prime}\left( x_{i} \right)}}}{1 + \frac{Q_{z\; 1}}{Q_{z\; 0}} - {\frac{1}{Q_{z\; 0}}{\sum\limits_{i = 1}^{N}{w_{i}\frac{V_{z\; 1}\left( x_{i} \right)}{V_{z\; 0}^{\prime}\left( x_{i} \right)}}}}}.}$
 8. The apparatus as defined in claim 7, wherein the Gaussian quadrature operation section is configured to use the V_(z1)(x) selected to allow V′_(z0)(x) to be always zero or more on the flow passage cross-section S, or to be always zero or less on the flow passage cross-section S.
 9. The apparatus as defined in claim 6, wherein the V_(z)(x_(i)) determining section comprises: a pair of ultrasonic elements, each placed at a position corresponding to each node x_(i) on a flow passage pipe arrangement defining the flow passage; an ultrasonic wave transmission and receipt circuit configured to give an instruction of transmission and receipt of ultrasonic wave to the pair of ultrasonic elements; an ultrasonic wave transmission and receipt signal recording circuit configured to record an ultrasonic wave transmission and receipt-indicative signal indicative of transmission and receipt of the ultrasonic wave; a flow velocity calculation circuit configured to calculate V_(z)(x_(i)) for each node x_(i) using the ultrasonic wave transmission and receipt-indicative signal recorded on the ultrasonic wave transmission and receipt signal recording circuit.
 10. The apparatus as defined in claim 7, wherein the V_(z)(x_(i)) determining section comprises: a pair of ultrasonic elements, each placed at a position corresponding to each node x_(i) on a flow passage pipe arrangement defining the flow passage; an ultrasonic wave transmission and receipt circuit configured to give an instruction of transmission and receipt of ultrasonic wave to the pair of ultrasonic elements; an ultrasonic wave transmission and receipt signal recording circuit configured to record an ultrasonic wave transmission and receipt-indicative signal indicative of transmission and receipt of the ultrasonic wave; a flow velocity calculation circuit configured to calculate V_(z)(x_(i)) for each node x_(i) using the ultrasonic wave transmission and receipt-indicative signal recorded on the ultrasonic wave transmission and receipt signal recording circuit.
 11. The apparatus as defined in claim 6, further comprising an error range estimating section configured to estimate an error range of the approximate value Q_(N) based on the following Formula using a second approximate value Q_(SUB) that is less reliable than the approximate value Q_(N): ${{{if}\mspace{14mu} Q_{SUB}} < Q_{N}},{\frac{Q_{SUB} - Q_{N}}{2} < {Q - Q_{N}}}$ ${{{if}\mspace{14mu} Q_{SUB}} > Q_{N}},{\frac{Q_{SUB} - Q_{N}}{2} > {Q - {Q_{N}.}}}$
 12. The apparatus as defined in claim 7, further comprising an error range estimating section configured to estimate an error range of the approximate value Q_(N) based on the following Formula using a second approximate value Q_(SUB) that is less reliable than the approximate value Q_(N): ${{{if}\mspace{14mu} Q_{SUB}} < Q_{N}},{\frac{Q_{SUB} - Q_{N}}{2} < {Q - Q_{N}}}$ ${{{if}\mspace{14mu} Q_{SUB}} > Q_{N}},{\frac{Q_{SUB} - Q_{N}}{2} > {Q - {Q_{N}.}}}$ 